Active Member

Was just playing with my new lathe and practicing threading on some .250 steel stock. I was running 20 TPI setting. I micd the stock and it read .250. I threaded great and it worked well. Problem is I micd after threading and the .250 stock now read .273.

So the question is..What is the rule of thumb of stock to start with given you know the final diameter? TIA...

H-M Supporter - Premium Member

.250 diameter. What you have caused is a burr on top of your thread you cut when using a die nut or single pointing a thread. Go over it with a flat file and remove the burr and recheck. Should now read .250". May also need to use a triangle file to break the edges of the freshly cut thread, too. A good cutting oil will help, too. Ken

Former Member

How did it get bigger? Is that what you are asking, if so it is the burr.

As far as starting diameter begin with the what the thread specifications will tell you which will be at maximum the Major Diameter of the thread for certain classes of thread fit, mostly somewhat smaller however.

Active User

Since common 60 deg. external threads have a flat or rounded top you do not have to cut until the tops are sharp.
Burrs will not be so high if you leave a small flat on top. Thread charts will give you the standard tolerances for diameters of various threads. For example, a 1/4-20 class 1A external thread can be as small as .237'' in o.d.

Active Member

You can find the recommended initial diameters in the machinest handbook if you happen to have one.
The initial diameter is usually a small amount less than the nominal bolt diameter.
If you are not really picky you can just keep cutting until the nut fits.
This works astonishing well if you are patient and make the last cuts very small.
However, I am certain that a REAL machinest would have heart failure upon reading this.

Former Member

Not at all, if making a threaded part that someone else will measure then by all means make it within the stated class fit or particular given dimensions.

If making something for personal use then do it however you want, one may also make non standard threaded parts that will confuse the hell out of someone in the future, 1/4-19, 5/16-15, 3/8-17 and so on.

Active Member

If making something for personal use then do it however you want, one may also make non standard threaded parts that will confuse the hell out of someone in the future, 1/4-19, 5/16-15, 3/8-17 and so on.

I'm guilty of doing the same thing! I find the looks on people's faces to be funny when they can't find a nut or a bolt that fits my weird choice of thread for a given diameter. I only do it for laughs then eventually tell them.